1. Field of the Invention
The present invention relates to methods and apparatus for coating high viscosity materials.
2. State of the Art
In recent years, workers in the battery art have begun to understand and recognize the advantages of so-called laminate batteries that include solid polymer electrolytes and sheet-like anode and cathodes. The advantages of such batteries include lower battery weights than conventional batteries that employ liquid electrolytes, longer service life, and relatively high power densities. The advantages of laminate batteries also include relatively high specific energies, and the elimination of the danger associated with batteries containing spillable liquid electrolytes such as acids.
Laminate batteries have been developed which possess good performance characteristics. For example, in U.S. Pat. No. 4,925,751, a laminate battery in which a cathode material formed from a mixture of an active cathodic material (preferably vanadium oxide V.sub.3 O.sub.8 of V.sub.6 O.sub.13), a conductive filler material (preferably carbon particles or filaments), and ionically conductive polymer electrolyte material is laminated on a conductive substrate material such as a nickel or copper web or sheet. A layer of polymer electrolyte material is laminated over the laminated layer of cathode material, and an anode material is applied over the laminated layer of polymer electrolyte material.
The cathode layer thickness is largely determinative of the discharge rate of the battery. The electrolyte material is applied over the cathode material by conventional coating techniques, such as with a doctor blade arrangement. The electrolyte material layer on the cathode is applied in the form of a curable, viscous, liquid and forms a layer that is generally between 5 and 50 microns thick.
The cathode material layer on the substrate generally has a thickness between 25 and 250 microns. While it is desirable to maintain as thin a layer of electrolyte material over the cathode material as possible to minimize the impedance of the electrolyte layer, if the surface of the cathode material layer is irregular and contains peaks and valleys, the average mean thickness of the electrolyte material layer must be increased to avoid excessively thin spots in the electrolyte material layer which may result in battery malfunction or failure. Good battery performance requires that the cathode layer and the electrolyte layer have constant thicknesses, with only very small tolerances being permitted. Further, construction of various battery configurations is greatly facilitated by maintaining constant thickness cathode and electrolyte material layers.
Like the electrolyte material layer, the cathode material is also applied on the substrate by conventional coating techniques, such as with a doctor blade arrangement or through an extrusion method. The cathode material is, however, a much more viscous material than the electrolyte material, often having a viscosity on the order of 1,000,000 centipoise. Moreover, the cathode material is a shear-thinning material, i.e., the viscosity of the material decreases with increasing shear.
In coating the cathode material onto a substrate, various problems with conventional coating techniques, such as extrusion, calendering, or doctor blade and roller techniques, have been identified. For example, conventional extruding apparatuses are often not adapted to accommodate shear-thinning materials. Such extruding apparatuses often include comparatively large reservoirs between comparatively small mixing chambers and extrusion nozzles. Such apparatuses are poorly suited to the shear-thinning cathode material. Large amounts of the cathode material that fills the volume of the reservoir often remains in the reservoir in a substantially stagnant condition. As additional cathode material enters the reservoir from the mixing chamber on its way to the extrusion nozzle, control of the density of the cathode material is difficult as the cathode material may expand into the reservoir or push material already in the reservoir out to the extrusion nozzle.
When cathode material is extruded from an extrusion nozzle, the cathode material, which is ordinarily under great pressure, will generally expand. Upon expanding, stray pieces of the cathode material often become "hung up" on lips of the nozzle. Such pieces of cathode material impact the subsequent extrusion by causing "striations", or streaks, in the extrusion, or they fall onto the extrusion as particles and cause lumps. Such problems are particularly acute when a lip of the extrusion nozzle serves both as an extrusion nozzle and as a doctor blade, i.e., the distance of the lips of the extrusion nozzle relative to the substrate receiving the extrusion, or a roller around which the substrate extends, is controlled so that a lip of the extrusion nozzle serves to control the thickness of the extrusion.